(1) Field of the Invention
The present invention relates to an aircraft, in particular a rotary-wing aircraft, provided with a device for reducing vibration, and to a method therefor.
The invention thus lies within the limited technical field of devices for reducing aircraft vibration.
(2) Background Art
Among aircraft, there can be distinguished, in particular, aircraft provided with at least one rotary wing connected to a load-bearing structure, which load-bearing structure is usually referred to as the “airframe” or as the “fuselage”.
Such an aircraft also has a powerplant that drives a main gearbox secured to the load-bearing structure of the aircraft. The main gearbox then includes a mast that drives the rotary wing in rotation.
It can be noted that the main gearbox is often connected to the load-bearing structure via an end wall and via auxiliary fastening means generally comprising three or four suspension bars. The structure for mounting the rotary wing and including the main gearbox and the suspension bars is sometimes referred to as the “pylon” by the person skilled in the art, because of its shape.
The rotary wing and/or the main gearbox can give rise to vibration that can adversely affect the comfort of the occupants of the cabin by generating vibratory movements and noise inside said cabin. In addition, items of equipment of the aircraft that are arranged inside the load-bearing structure can be deteriorated by the vibration generated by the assembly comprising the main gearbox and the rotary wing.
Various devices are known for at least reducing the vibration felt in the cabin in order to improve the comfort of the occupants of said cabin.
In a first solution, it is possible to provide passive devices acting at a predetermined frequency. Although appealing, that solution has limitations if the vibration to be reduced changes frequency.
In a second solution, it is possible to use active devices in the cabin. That solution suffers from the drawback of acting only on vibration that is present in the cabin.
In a third solution, devices are arranged in the form of pendulum dampers acting on the rotary wing. That solution aims to act on the vibration at source, but it suffers from the drawback of acting on parts of the aircraft that are essential for its operation.
Reference can be made, for example, to the book by François Malburet and Tomasz Krysinski entitled “Mechanical Vibrations: Active and Passive Control” and published by ISTE).
More particularly, Document FR 2 731 405 suggests fastening a main gearbox to a helicopter load-bearing structure via resilient connection devices controlled via a computer as a function of information received by sensors arranged on the fuselage, in order to minimize dynamic excitation exerted at the center of gravity of the fuselage. Such a resilient connection device may comprise a cylinder defining two chambers separated by a piston. That piston has a rod connected to the main gearbox, while a bar secured to the cylinder is connected to the load-bearing structure. In addition, with the chambers of the cylinder being interconnected via a pipe, it is possible to control the resilient connection device by controlling the head loss generated by said pipe.
Similarly, instead of providing suspension bars per se, Document EP 0 501 659 provides actuators that are formed integrally with mechanical struts so as to interconnect a main gearbox and a load-bearing structure.
Document FR 1 506 385 mentions the use of suspension bars and of a bar including a resilient connection associated with an excitation jack.
Document FR 2 770 825 proposes to arrange two-stage resonators in the cabin at the seats of an aircraft for the purpose of at least reducing the vibration felt in said cabin, each two-stage resonator being controlled by a control unit implementing a plurality of successive operations.
Each two-stage resonator comprises a first resonator presenting a predetermined resonance frequency, and a second resonator associated with the first resonator, the second resonator being frequency-adjustable by means of an actuator.
That architecture is, in particular, applied to the helicopter EC 725® MKII+.
Document FR 2 566 862 presents an aircraft provided with a powerplant and with a main gearbox that are disposed on a raft, actuators being disposed between the raft and the fuselage of the aircraft.
Document U.S. Pat. No. 5,310,137 presents an active noise control system provided with beams of a load-bearing structure, those beams including actuators.
Document EP 0 541 277 presents actuators for inputting forces into a structure.
Also known is the Document TROCKMORTON ALEXANDER: “HELITECH 91\REVIEW”, AIRCRAFT ENGINEERING, BUNHILL PUBLICATIONS LTD. LONDON, GB, vol. 63, No. 11, Nov. 1, 1991 pages 12-14.